MeiosisMeiosis

Division of Sex CellsDivision of Sex Cells

MeiosisMeiosis A process of cell division in which the number of A process of cell division in which the number of

chromosomes per cell is cut in half through the chromosomes per cell is cut in half through the separation of homologous chromosomes in a separation of homologous chromosomes in a diploid cell; formation of gametesdiploid cell; formation of gametes

DiploidDiploid – 2 sets of chromosomes (2 copies of – 2 sets of chromosomes (2 copies of each)each)

HaploidHaploid – 1 set of chromosomes (1 copy of each) – 1 set of chromosomes (1 copy of each) HomologousHomologous – chromosomes that each have a – chromosomes that each have a

corresponding chromosome from the opposite sex corresponding chromosome from the opposite sex parent (1 from father, 1 from mother)parent (1 from father, 1 from mother)

Meiosis StagesMeiosis Stages Meiosis usually involves 2 distinct stagesMeiosis usually involves 2 distinct stages

Meiosis IMeiosis I Meiosis IIMeiosis II

Prophase I (Meiosis I)Prophase I (Meiosis I) Each chromosome pairs with its corresponding Each chromosome pairs with its corresponding

homologous chromosome to form a tetrad.homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad.There are 4 chromatids in a tetrad. Crossing-over may occur hereCrossing-over may occur here Crossing-over is when chromosomes overlap Crossing-over is when chromosomes overlap

and exchange portions of their chromatids.and exchange portions of their chromatids.

Prophase IProphase I

Metaphase IMetaphase I Spindle fibers attach to the chromosomesSpindle fibers attach to the chromosomes Chromosomes line up in the middle of Chromosomes line up in the middle of

the cellthe cell

Metaphase IMetaphase I

Anaphase IAnaphase I The fibers pull the homologous The fibers pull the homologous

chromosomes toward opposite ends of chromosomes toward opposite ends of the cell.the cell.

Anaphase IAnaphase I

Telophase I & CytokinesisTelophase I & Cytokinesis Nuclear membranes form.Nuclear membranes form. The cell separates into 2 cells.The cell separates into 2 cells.

Telophase ITelophase I

Prophase II (Meiosis II)Prophase II (Meiosis II) Meiosis I results in two haploid (N) cells.Meiosis I results in two haploid (N) cells. Each cell has half the number of Each cell has half the number of

chromosomes as the original cell.chromosomes as the original cell.

Prophase IIProphase II

Metaphase IIMetaphase II The chromosomes line up similar to The chromosomes line up similar to

metaphase in mitosis.metaphase in mitosis.

Metaphase IIMetaphase II

Anaphase IIAnaphase II Sister chromatids separate and move to Sister chromatids separate and move to

opposite ends of the cell.opposite ends of the cell.

Anaphase IIAnaphase II

Telophase IITelophase II Meiosis II results in 4 haploid cells.Meiosis II results in 4 haploid cells.

Telophase IITelophase II

Gamete FormationGamete Formation In males, meiosis results in 4 sperm cellsIn males, meiosis results in 4 sperm cells In females, meiosis results in 1 egg cell In females, meiosis results in 1 egg cell

and three polar bodies, which are not and three polar bodies, which are not used in reproduction.used in reproduction.

Mitosis vs MeiosisMitosis vs Meiosis

MitosisMitosis MeiosisMeiosisResults inResults in 2 Diploid Cells 2 Diploid Cells

(2N)(2N)4 Haploid Cells 4 Haploid Cells (N)(N)

Cells areCells are Genetically Genetically IdenticalIdentical

Genetically Genetically DifferentDifferent

Occurs inOccurs in Somatic (Body) Somatic (Body) CellsCells

Sex CellsSex Cells